1. Field of the Invention
Embodiments of this invention relate to the compensation of errors in the rotor position detector of an electrical machine and particularly, but not exclusively, of a switched reluctance machine.
2. Description of Related Art
The characteristics and operation of switched reluctance systems are well known in the art and are described in, for example, “The characteristics, design and application of switched reluctance motors and drives” by Stephenson and Blake, PCIM'93, Nürnberg, 21–24 Jun. 1993, incorporated herein by reference. A general treatment of the drives can be found in various textbooks, e.g. “Electronic Control of Switched Reluctance Machines” by T J E Miller, Newnes, 2001, incorporated herein by reference.
FIG. 1 shows a typical switched reluctance drive in schematic form, where the switched reluctance motor 12 drives a load 19. The input DC power supply 11 can be either a battery or rectified and filtered AC mains. The DC voltage provided by the power supply 11 is switched across the phase windings 16 of the motor 12 by a power converter 13 under the control of the electronic control unit 14. Current sensor 18 determines current in at least one of the phases.
The switching must be correctly synchronized to the angle of rotation of the rotor for proper operation of the drive. A rotor position transducer (‘rpt’) 15 is typically employed to supply signals corresponding to the angular position of the rotor. The rpt 15 is a device that outputs a binary signal having two transitions per machine phase period and is periodic with an electrical cycle of the machine. The transitions are indicative of events in the electrical cycle of the machine, for example occurrences of maximum and minimum inductance, or positions closely adjacent such events, in relation to which a control action is to take place.
Typically, a set of control laws is programmed into the control unit 14 and these laws are used to operate the drive in response to user demands such as speed or torque. The laws are frequently written in terms of control angles, e.g. an angle at which excitation is applied to a phase winding; a second angle at which the excitation is removed from the phase; and a third angle describing the duration of any freewheeling period used. While techniques exist for determining these laws empirically for each drive during commissioning, it is more common for a prototype drive to be tested in detail, the control laws determined, and these laws programmed into successive models of the drive, on the assumption that the drives are sufficiently similar that the small differences in performance are insignificant. This procedure does, however, rely on the assumption that the rpt of each drive is accurately built and aligned.
Many different power converter topologies are known, several of which are discussed in the Stephenson paper cited above. One of the most common configurations is shown for a single phase of a polyphase system in FIG. 2, in which the phase winding 16 of the machine is connected in series with two switching devices 21 and 22 across the busbars 26 and 27. Busbars 26 and 27 are collectively described as the “DC link” of the converter. Energy recovery diodes 23 and 24 are connected to the winding to allow the winding current to flow back to the DC link when the switches 21 and 22 are opened. A resistor 28 is connected in series with the lower switch 22 to provide a current feedback signal. A capacitor 25, known as the “DC link capacitor”, is connected across the DC link to source or sink any alternating component of the DC link current (i.e. the so-called “ripple current”) which cannot be drawn from or returned to the supply. In practical terms, the capacitor 25 may comprise several capacitors connected in series and/or parallel and, where parallel connection is used, some of the elements may be distributed throughout the converter. A polyphase system typically uses several “phase legs” of FIG. 2 connected in parallel to energize the phases of the electrical machine. Instead of the current-measuring resistor, an isolated and/or non-invasive current detector may be used.
The phase inductance cycle of a switched reluctance machine is the period of the variation of inductance for the, or each, phase, for example between maxima when the rotor poles and the relevant respective stator poles are fully aligned. An idealized form of the inductance curve for a phase is shown in FIG. 3(a). In practice, the sharp corners at Lmin and Lmax are rounded due to flux fringing and to saturation of the magnetic circuits. The maximum value of inductance would also be current dependent. Nevertheless, this curve is useful to illustrate the general behavior of the machine. As explained in more detail in the Stephenson paper cited above, the maximum inductance region, Lmax, is centered around the rotor position where a pair of rotor poles are fully aligned with a pair of stator poles. This is shown for a 3-phase, 6-pole stator, 4-pole rotor machine in FIG. 3(b). Similarly, the minimum inductance region, Lmin, corresponds to the position where the interpolar axis on the rotor is aligned with the stator pole axis, as shown in FIG. 3(c).
The performance of a switched reluctance machine depends, in part, on the accurate timing of phase energization with respect to rotor position. Detection of rotor position is conventionally achieved by using a rotor position transducer 15, shown schematically in FIG. 1, such as a rotating toothed disc mounted on the machine rotor, which co-operates with an optical or magnetic sensor mounted on the stator. A pulse train indicative of rotor position relative to the stator is generated and supplied to control circuitry, allowing accurate phase energization. Typically, a single sensor is used for 1- and 2-phase systems; three sensors for a 3-phase system; and either 4 or 2 sensors for a 4-phase system. Simpler arrangements using only one sensor are occasionally used in systems with three or more phases. Such position transducers have a much poorer resolution than, say, a resolver or encoder but are considerably less costly. While it is possible to use highly accurate sensors, the cost involved would have an impact on the overall cost of the drive, particularly in small, low-cost drives.
FIG. 4 shows in schematic form the essential components of such a rotor position transducer (rpt) for a 3-phase system. The vane 40 is proportioned so as to give an equal mark:space ratio on the outputs of the three sensors. The sensors are distributed around the perimeter of the vane at angles which correspond to the displacement angles of the inductance profiles of the phases, and are typically set relative to the stator poles to give rising and falling edges at Lmin and Lmax, respectively. This results in the signals from the sensors having relationships with the inductance profiles of the phases as shown in FIG. 5. As stated above, the rpt 15 is a device that outputs a binary signal having two transitions per machine phase and is periodic with an electrical cycle of the machine. The transitions are indicative of events in the electrical cycle of the machine, for example occurrences of maximum and minimum inductance, or positions closely adjacent such events, in relation to which a control action is to take place. These signals are typically used by the control system to generate the correct instants for energization of the windings of the machine in accordance with the predetermined control laws. Since the performance of the machine is critically dependent on the accuracy of such energization, it is important that the components of the rpt are accurately made and aligned.
Several sources of error are commonly found in the rpt. The mark:space ratio of the vane obviously affects the mark:space ratio of the output signal, though the relationship is not entirely straightforward, since it is also affected by the properties of the type of sensor used in the rpt. For example, if the sensor is of the optical type, it will have a finite beam width. This will influence the signal differently, depending on whether the transition is from light transmitting to light blocking or vice versa. If the sensor is of the Hall-effect type, then the proximity of the incoming edge of the ferromagnetic vane will give rise to fringing of the magnetic flux and earlier switching than would be expected. In addition, both of these types of sensor can suffer from hysteresis effects, giving variations in signal output depending on the direction of rotation. In order to counter these effects, it is known to adjust the physical mark:space ratio of the vane so as to give a sensor output which is more nearly unity mark:space. It is also known to offset the alignment of the vane on the rotor in order to at least partially compensate for hysteresis, magnetization precision, beam-width and/or fringing effects. Nevertheless, it is not usually possible to compensate simultaneously for all the errors, so at least some errors usually remain in the output signals.
These errors, however, are only part of the problem. It will be evident from FIG. 4 that both the absolute position of a sensor relative to the stator and its relative position to the other sensors will affect the phase of the RPTA, RPTB and RPTC signals relative to the inductance profile of their phase. Methods have therefore been developed to reduce manufacturing errors in the placement of the sensor components, which are normally arrayed on a printed circuit board. For example, U.S. Pat. No. 5,877,568 and U.S. Pat. No. 6,661,140, both incorporated herein by reference, both disclose methods of improving the alignment of the sensors with the stator, though at the expense of additional components and manufacturing processes.
Similarly, the alignment of the vane relative to the rotor poles affects the phase relationship of the rpt signals with the respective inductance profiles. Among known methods for reducing this error is that disclosed in U.S. Pat. No. 5,786,646, incorporated herein by reference, which uses a specially designed clamp ring and appropriate tooling to fix the vane in a known relationship with the rotor poles.
These methods, while going at least some way to improving the quality of the rpt output, are expensive in terms of added components, manufacturing processes and/or set-up costs. While this may be acceptable in high-value drives manufactured in low volumes, it is not desirable for low-cost, high-volume drives as used in, e.g., domestic appliances or automotive systems. Nevertheless, such low-cost systems still require accurate rpt signals to produce the high output required of them. There is therefore a need for a method of compensating for the error in the rpt signals in a repeatable and cost-effective way.